I'm trying to write a Java function that takes a singly linked list: a->b->a->b->a->b->a->null, and groups both even and odd nodes based on position (not value). The function should work in-place and return all even nodes first, followed by odd nodes: b->b->b->a->a->a->a->null. I currently have the following solution:

However, this produces the error: Cannot assign field "next" because "even" is null. Could you please help fix this? The funtion must make all even nodes first, followed by all odd nodes.